1. Field of the Invention
The present invention relates generally to the field of communications systems. In one aspect, the present invention relates to a circuit and method of generating modulated radio frequency (“RF”) signals having an interleaved IQ modulation.
2. Description of the Related Art
Modern communication systems, such as cordless telephones, pagers, cellular telephones, and global positioning systems, generally communicate using radio frequency (RF) signals. Voice/data input into a cellular telephone is a low frequency or baseband signal. A transmitter converts the baseband signal to a radio frequency signal for transmission by the cellular telephone. The transmitter typically includes a low frequency data modulation stage, one or more frequency conversion stages and a high frequency power amplifier and transmission stage. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. For example, the baseband signal can be generated using any of a variety of modulation techniques, such as quadrature modulation (also known as IQ modulation and Quadrature Amplitude Modulation (QAM)) which is a technique used with modern communication systems which enables two independent signals to be combined at a transmitter, transmitted on the same transmission band, and separated at a receiver. The principle of quadrature modulation is that two separate signals, I and Q (In-phase and Quadrature phase), are modulated by using the same carrier wave frequency, but the carrier wave of signal Q is ninety degrees out of phase with the carrier wave of signal I. Conventionally, two modulation inputs are applied to two separate modulators, each of which is supplied with sine or cosine carriers, respectively. At the frequency conversion stage, the baseband signal output of the modulators are mixed with one or more local oscillations to produce RF signals containing the I and Q information. While conventional mixers algebraically combine the RF signals to create a single RF transmit signal, switched-mode mixers or modulators have been shown to present improved noise, linearity, and current-drain performance. For example, U.S. Patent Publication No. US2007/0201581/A1 to Wagh et al. entitled “RF Transmitter with Interleaved IQ Modulation” describes an RF modulator mixer which interleaves in-phase and quadrature signals. However, there remain design challenges with the operation of switched mode mixers in terms of suppressing carrier and sideband frequencies. For example, when switched-mode mixers require interleaved local oscillator signals as inputs, the performance of the mixer is impaired if there are propagation delays in the generated local oscillator signals. With higher frequency transmitters, the propagation delay issue becomes more problematic.
Accordingly, there is a need for an improved transceiver circuit to overcome the problems in the art, such as outlined above. Further limitations and disadvantages of conventional processes and technologies will become apparent to one of skill in the art after reviewing the remainder of the present application with reference to the drawings and detailed description which follow.